Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as an energy source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several small-sized unit cells for each device. On the other hand, medium- or large-sized devices, such as vehicles, use a medium- or large-sized battery module having a plurality of unit cells electrically connected with each other because high output and large capacity are necessary for the medium- or large-sized devices.
Based on their external shape, the unit cells are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. The pouch-shaped battery has been increasingly used since the pouch-shaped battery can be stacked with high integration, the shape of the pouch-shaped battery can be easily modified, the weight of the pouch-shaped battery is small, and the manufacturing costs of pouch-shaped battery are low.
Generally, the pouch-shaped battery is manufactured by placing an electrode assembly including cathodes, anodes, and separators in a pouch-shaped sheet and sealing the pouch-shaped sheet. The pouch-shaped battery includes electrode terminals constructed by welding pluralities of electrode taps protruding from electrode collectors to corresponding electrode leads. When a battery module is manufactured using pouch-shaped batteries as unit cells, electrode terminals of the pouch-shaped batteries are welded to bus bars to accomplish the electrical connection between the pouch-shaped batteries.
FIG. 1 is a typical view illustrating a part of a process for manufacturing a battery module using pouch-shaped batteries according to a conventional art.
Referring to FIG. 1, a cathode terminal 4 of a pouch-shaped battery 1 is connected to an anode terminal 5a of a neighboring pouch-shape battery 1a via a bus bar 6 so as to accomplish the electrical connection between the pouch-shaped batteries 1 and 1a. The cathode terminal 4 of a pouch-shaped battery 1 is constituted by a cathode lead welded to cathode taps 3 protruding from a cathode collector 2. In the pouch-shaped battery 1 with the above-described structure, the cathode lead 4 has the same purpose as the cathode lead welded to the cathode taps 3. Accordingly, the electrode terminals will be also referred to as the electrode leads in the following description.
The pouch-shaped batteries 1 and 1a must be electrically connected with each other so as to manufacture a battery module. To this end, the bus bar 6 is connected to the cathode terminal 4 of the pouch-shaped battery 1 and the anode terminal 5a of pouch-shaped battery 1a by welding. It is preferable that the electrical connection between the pouch-shaped batteries be accomplished by welding since the contact resistance between the electrode terminals 4 and 5a and the bus bar 6 is minimized.
Generally, the cathode terminal 4 of the pouch-shaped battery 1 is made of aluminum. The anode terminal 5a of pouch-shaped battery 1a is made of copper. The bus bar 6 is mainly made of copper. However, welding between different materials is not as satisfactory as compared to welding between the same materials. This is because the interior of the battery is damaged due to heat generated during welding, or the quality of welding is deteriorated.
In order to solve the above-mentioned problem, some conventional arts have proposed technologies for performing welding, while materials having high thermal conductivity are temporarily in contact with the lower ends of regions where the electrode leads 4 and 5a are connected with the bus bar 6 by welding, such that high heat generated from the welding regions is prevented from being conducted into the battery 1. However, these technologies complicate a process for assembling a battery module, which increases the manufacturing costs of the battery module.